Hip osteoarthritis is a major cause of disability in middle aged and older persons. After the knee, it is the most common reason for joint replacement surgery. Despite this impact, relatively few risk factors, and fewer still potentially modifiable ones, for hip OA development and progression have been identified. Severe developmental abnormalities of the acetabulum and proximal femur are known to confer a high risk for developing hip OA and joint failure at a relatively young age. There is increasing evidence that more subtle variation in the morphology of the acetabulum and proximal femur, including mild forms of developmental abnormalities, increase the risk of hip OA later in life. Recent orthopedic research has focused on variations of hip morphology that alter hip joint mechanics and joint surface stress, in particular those related to femoro-acetabular impingement. It is hypothesized that these are important, but previously unrecognized, risk factors that may account for a large proportion of hip OA occurrence in mid and late life. Some of these hip morphological variations are modifiable through surgical interventions, and there is increasing interest in early treatment to prevent later OA. To date, no large prospective cohort studies have comprehensively tested whether these variations, alone or in combination, are related to the risk of development and progression of radiographic hip OA (RHOA). The Osteoarthritis Initiative (OAI) is well known as a public access research database from the largest prospective study of biomarkers (MRI, biochemical, genetic) of knee OA incidence and progression. It is less well known that the OAI is also a prospective study of hip OA. Pelvic radiographs were obtained in the entire cohort of 4796 participants at baseline and in the 80% of subjects projected to attend the 4-year follow-up clinic visits. We propose to assess the OAI pelvic radiographs for prevalent, progressive and incident RHOA using validated scoring systems and measurements and to rapidly release the results of these readings through the OAI public web site, thus making it possible for hip OA presence and outcomes to be included in biomarker studies using the OAI cohort. We also propose to comprehensively assess the baseline pelvic radiographs for measures of hip morphology hypothesized to play a role in the development of hip OA, and to test the effect of these measures as risk factors for incident and progressive RHOA in this large, well-characterized cohort. Hip morphology will also be measured on follow-up images to allow investigation of change from baseline in these parameters as imaging biomarkers of RHOA